Octahedral complexes acid hydrolysis

Acid hydrolysis of an octahedral metal ion complex is typically a dissociative or SNl-type reaction. In the case of base hydrolysis, reactions tend to display SN2-type reaction mechanisms, although others take place by what is termed an SnI-conjugate base mechanism. The latter involves attack by an electrophile to abstract a proton... 

Hydroxide ion also may have an appreciable effect on the rate of hydrolysis of octahedral complexes. The rate constant for hydrolysis of [Co(NH3)5CllJ in basic solution is a million times that found for acidic solutions. Furthermore, the reaction is found to be second order and dependent on tbe hydroxide ion concentration ... 

Complexation228 of Zn2+ with the quadridentate 1,5-diazacyclooctane-N, A -diacetic acid (H2L) leads to formation of the trigonal prismatic [Zn(L)H20] species. The hydrolysis constant of the coordinated H20 (pKa = 8.6) is much lower than that of the coordinated H20 in the octahedral complex formed by ethylenediaminediacetate (pXa = 10.5). This increased acidity of coordinated H20 with decreasing coordination number has also been previously noted in the [Zn(Me6tren)H20]2+ cation,229 and has been attributed in part to the hydrophobic environment provided by the Me groups. This finding has important implications for the role of the... 

Although aquation (or acid hydrolysis) reactions of cobalt(III) complexes have been studied more extensively than any other octahedral substitutions, it still is impossible to assign detailed mechanistic paths for them. An aquation reaction takes place at a pH less than 4 and is one in which a ligand is replaced by a molecule of water, such as that shown by Equation 1. 

The reactions to illustrate weathering of complex silicates during acid hydrolysis (eqns. 4.13 4.14) predict that clay minerals will be an important solid product and this is confirmed by looking at soils. Clay minerals are important constituents in most soils. These sheet silicates that are less than 2 pm (Section 4.2.3) are constructed of layers of atoms in tetrahedral and octahedral coordination, known as tetrahedral and octahedral sheets. 

Why do rate laws fail to give information about the mechanism of acid hydrolysis in octahedral complexes ... 

Lower oxidation states are rather sparsely represented for Zr and Hf. Even for Ti they are readily oxidized to +4 but they are undoubtedly well defined and, whatever arguments may be advanced against applying the description to Sc, there is no doubt that Ti is a transition metal . In aqueous solution Ti can be prepared by reduction of Ti, either with Zn and dilute acid or electrolytically, and it exists in dilute acids as the violet, octahedral [Ti(H20)6] + ion (p. 970). Although this is subject to a certain amount of hydrolysis, normal salts such as halides and sulfates can be separated. Zr and are known mainly as the trihalides or their derivatives and have no aqueous chemistry since they reduce water. Table 21.2 (p. 960) gives the oxidation states and stereochemistries found in the complexes of Ti, Zr and Hf along with illustrative examples. (See also pp. 1281-2.)... 

An analysis of the RDF for the acid solution shows that the In3+ ion is bonded to six water molecules at 2.17 A (Fig. 28). The same In—H20 distance is found for octahedrally coordinated In3+ in crystal structures (223, 224). According to the difference curve this coordination is not changed by the hydrolysis. A possible model for a tetranuclear complex with four octahedrally coordinated In atoms occupying the four corners of a regular tetrahedron and joined by single hydroxo bridges is shown in Fig. 28. It is consistent with the experimental data and seems to be a likely model for the hydrolysis complexes formed in solution, but has not yet been found in crystal structures. 

Apart from Pd(N03)2(0H)2 formed on dissolution of Pd in concentrated nitric acid, the complexes are mainly the octahedral halide anions, MX. The fluoro complexes of Ni, Pd, and Pt are all very similar and are rapidly hydrolyzed by water. The chloro and bromo ions are stable to hydrolysis but are decomposed by hot water to give the Pdm complex and halogen. The red PdCl ion is formed when Pd is dissolved in aqua regia or when PdCl4 solutions are treated with chlorine. 

In very acidic solutions, bismuth(III) exists in the form of the nonaaquo ion [Bi(H20)9] +, which is similar to the aquo complexes of the lanthanide ions, but partial hydrolysis of bismuth(III) salts leads to the formation of bismuth oxo clusters. The core structure of these complexes is often based upon a Bie octahedral core with oxide, hydroxide, or alkoxide functions bridging the edges and/or faces of the octahedron. The [Bi6(OH)i2] + ion (11) has been studied spectroscopically. In oxo clusters, the octahedron is face-bridged by eight oxo or alkoxide functions (12). Such core structures have been found in the hydrolysis of bismuth nitrate or perchlorate. ... 

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